Photoelectric apparatus



Aug. 14, 1934. P. T. FARNswoRTH 1,970,036

` PHOTO ELECTRIC APPARATUS Filed aan. 9, 192s HIS ATTOle/YEY PatentedAug. 14, 1934 UNITEDv STATES PATENT OFFICE by mesne assignments, to

Francisco, Calif., a corporatories, Inc., San tion of CaliforniaTelevision Labora- Application January 9, 1928, Serial No. 245,334 14Claims. (C1.25027.5)

My invention relates to television; and this application is acontinuation in part of my copending application, Serial No. 159,540;filed January '7, 1927. The broad object of the invention is to providea generator of picturemodulated electric currents.

An object of my invention is to provide a photo-electric tube in whichpicture analysis is 1o accomplished.

, Another object of my invention is to provide a photo-electric tube inVwhich the eiect of lightsensitive material other than that on thecathode is minimized. A further object is to provide a tube giving ahigh degree of definition. A

Another object is to provide a photo-electric tube in which a simpleoptical system may be used to focus an image directly upon the activelight-sensitive surface.

Still another object of my invention is to provide a photo-electricapparatus of high sensitivity.

My invention possesses other objects and valuable features, some ofwhich will be set forth in the following description of my inventionwhich is illustrated in the drawing forming part of the speciiication.It is to be understood that I do not limit myself to the showing made bythe said description and drawing, as I may adopt varying forms of myinvention within the scope of the claims.

Referring to the drawing:

Fig. 1 is a sectional view of my apparatus, the plane of section beingnormal to the cathode and parallel to the axis of the tube. The opticalsystem is shown schematically.

Fig. 2 is a sectional view on the line 2-2 of Fig. l. The optical systemis not shown in this figure.

Fig. 3 is a For clearance in ecting coils and omitted. 45 Broadlyconsidered, my invention comprlses a plane cathode coated with lightsensitive material, such aspotassium hydride, in an evacuated bulb.Placed as closely as is practical to the cathode is abox-or-prism-shaped hollow anode. The surface of the anode nearest thecathode` is of wire mesh or similar conductive material permeable tolight, and is parallel to the cathode. The opposite face is ot similarmaterial and is apertured at the center.

55 Behind this aperture, as viewed from the cathode, is an auxiliaryelectrode or target, which diagram of the electrical circuits.

the drawing one pair of detheir exciting oscillator are is completelyshielded from electron discharge with the exception of a single smallarea which isexposed to discharge passing thru thev aperture. Theauxiliary electrode is preferablyof nickel, or other material whichemits electrons readily under cathode ray bombardment.

Optical means external' to the tube form an image on the cathode,focusing it thru' the screen faces of the anode. As the elements ofthese screens lie without the focal plane they cast no 55j deiinedshadows on the cathode. An external source provides a positive potentialupon the anode, which accelerates the electrons liberated from thecathode by the light forming the vimage thereon. The major portion ofthese electrons pass thru the screen face of the anode to enter theequipotential space within it, traveling toward the opposite face atconstant velocity.

Means are provided for deilecting the electrons within the anode in twodimensions and in accordance with a predetermined time schedule, so asto pass every part of the beam successively over the aperture behindwhich is located the target.

The electrons entering the aperture impinge upon the target, causing itto liberate further electrons by secondary emission. An external circuitcontains a source of potential which maintains the auxiliaryelectrode'negative with respect to the anode, and the secondary emissionis thereby drawn to the anode, causing a current to flow in the externalcircuit. As this current corresponds in intensity to the illumination ofthat part of the cathode whose emission is at the instant falling uponthe aperture, it is in fact. a picture current, and may be used as suchin television systems.

In terms of greater detail a preferred embodiment of my inventionemploys an evacuated glass bulb or tube 6 having a stem 'I of the usualtype, a side seal 8, and the customary evacuating tip 9.

The side seal 8 comprises a short tube sealed into the side of the bulb.Within this tube is tted a roll 11 of spring tempered metal, usuallynickel, carrying a wire frame 12 which supports 10 the clips 13. Theseclips grasp the rectangle of plate glass 14, and make contact with asilver lm on which is deposited the light sensitive coating 16 whichactsas-the cathode of the tube. The entire structure is connected to itsexternal circuit by the wire 17 which is connected to the frame 12 andpasses thru the side seal.

The deposit of light sensitive material upon the cathode is accomplishedduring the process of evacuation in the customary manner. Metallicpotassium is -distilledl from a Ireservoir in the exhaust line, and isallowed to condensev on the cathode surface. The anode, and the walls ofthe bulb, except adjacent the cathode, are heated, so that littlecondensation takes place on these parts. Hydrogen is then admitted tothe tube, and in this atmosphere a glow discharge is passed betweencathode and anode, converting the potassium to the much morephoto-sensitive hydride.

The anode is supported by the stem 7. The wire lead 19 passing thru theseal, and the "dummy 18, which is held by the seal but does not passthru it, are bent to embrace the anode and are welded thereto. The anodeis preferably in the form of a box or rectangular prism. The oppositesides or walls 20 and 21 are of fine mesh screen, the side 20 beingparallel to the cathode and as close thereto as is structurallypractical. The two adjacent walls 22 are sheet metal. They arepreferably of low magnetic permeability and electrical conductivity tolimit eddy currents as far. as possible. Wire frames 23 and 24 aroundthe top and bottom of the anode give it rigidity.

Behindy the center of the screen 21 is the auxiliary electrode or target26 of nickel or other material which is a good secondary emitter ofelectrons. This is completely shielded with the lexception of the singlearea behind the small aperture 27 in the housing 28 which surrounds thetarget. yThe housing 28 is electrically connected to the metallic sheath29, surrounding the lead 31 from its connection with the electrode 26the light sensitive cathode.

not in thefocal plane they cast no discrete shadows, altho they dorintercept a certain proportion to its seal in the stem 7, and insulatedfrom the lead preferably by a tube 32 of boro-silicate glass. Theshielding system, comprising sheath and housingis connected to theanode.

Means are provided for deflecting the electrons emitted from thecathode. Coils 33 and 34, co-axial with the bulb, set up a magneticfield which bends the electron beam transversely of the tube. A secondset of coils 36 and 37, is parallel with the anode walls 22. Currentsflowing in these coils cause a. magnetic field which bends the beam in adirection parallel to the axis of the tube. By passing currents ofdifferent frequency thru the two sets of coils the beam may be oscil`lated so that each element of its cross section in turn traverses theaperture 2'7, as is described in my co-pending application No. 159,540above mentioned.` The oscillators 38, for accomplishing this may be ofany suitable character, altho I prefer to use the type there described.

A source of potential, conveniently the battery 41, imposes a highpositive voltage on the anode with respect to the cathode. I Theauxiliary electrode is maintained slightly negative to the anode, beingconnected to an intermediate point on the battery thru the highresistance 42.

An optical system, indicated by the lens 43,

focuses an image of the picture or view to be reproduced thru the anodescreens 21 and 20 upon As the screens are of the rays.

The cathode emits electrons from each elementary area of its surface ata rate proportional to the illumination of the area.` These electronsleave the cathode surface with small velocities in random directions.They immediately fall under the influence of the strong electrostaticfield from the anode, yvhich accelerates them in a direction normal tothe cathode surface until they pass thru the screen 20 as an electricalimage, or beam of electrons whose intensity in a plane normal 't0 thepath of the beam is at all points substantially proportional to thecorresponding point of the optical image. Beyond this screen'theycontinue to Atravel with the high velocity thus acquired. Whiletraveling at this velocity they are deflected by the fields from the twosets of coils 33, 34 and 36, 37, and are separated into two trains. Thefirst and by far the smaller of these trains comprises those enteringthe aperture 2'7 to strike the auxiliary electrode. The larger traineventually strikes either the wall of the bulb or the anode. This trainserves no further useful purpose.

The first train liberates a secondary electron discharge from thetarget. The electrons thus liberated are attracted to the more positiveanode or to the housing 28, carrying a current which flows thru aportion of the battery 4l and the high resistance 42 back to the target.This of course refers to the actual electron flow and not to theconventional current flow. There is thereby set up across the terminalsof the resistor 42 a potential which corresponds to an analysis, inaccordance with a predetermined time schedule, of the illumination ofthe cathode. An amplifier 44 connected across the resistor 42 isactivated by this potential and its output supplies the picture currentfor television transmiss1on.

The construction thus set forth has a number of highly advantageousfeatures. The target is shielded from all electron bombardment exceptthat originating from the cathode. This makes the accidental depositionof light-sensitive material on other parts of the tube relativelyunimportant. As such deposition is almost certain .to Occur to someextent this is a valuable property. The light-permeable anode screenspermit the image to be formed directly upon the active surface of thecathode, thus escaping many difficulties which are usually encountered.The use of 115 secondary emission gives a current which is from five totwenty times as great as can be obtained by photo-emission alone.

In addition to these advantages, my apparatus gives greatly improveddefinition. There are two causes for the electrons which leave a smallarea spreading to impinge upon a larger area of the target, causingblurring or diffusion. The first is their mutual repulsion. Where anarrow pencil is used as in the more usual type of apparatus, this hasits maximum effect. In my apparatus the space charge in the beam outsideof the portion whichr is momentarily active counteracts the chargewithin that portion, and largely negatives this effect. The second causeof diffusion is the 130 random initial velocity with which the electronsleave the cathode. 'I'his velocity in general has a component normal tothe desired electron path. To render the diffusion due to the normalcomponents negligible the velocity along the path must be made very highin comparison, and this is done by using a high anode potential. Thevelocity at which the electron travels is proportional to the potentialthru which it has fallen, and its mean velocity up to the time itreaches the anode is one half its final velocity. In my device the finalvelocity is acquired by the time the electron reaches the anode screen20, and its travel thru the substantially equipotential space betweenthe screens is at this final velocity. Its mean velocity from cathode totarget is therefore almost twice as great as it would be if the screen20 were omitted, and diffusion is therefore reduced to about half whatit would otherwise be. The result is the same as tho the anode voltage150 ode.

were almost doubled, and this without the manifest dangers that doublingthe voltage would involve.

In using the term electrically permeable" in this specification, I meanpermeable to cathode ray or electronic discharge, and do not refer toIclaim'L" i 1. A/phot'o-elect'rictube comprising an anode permeable tolight and a photo-sensitive cathode, and an auxiliary electrode havingall but a minute area shielded from said cathode and positioned toreceive a predetermined portion of the discharge therefrom.

2. A photo-electric tube having a cathode, and an anode comprisingscreens permeable to light and dening a substantially equipotentialspace.

3. A photo-electric tube having a cathode, and an anode comprisingparallel screens permeable to light and parallel to the cathode anddefining ,a substantially equipotential space. i

4. A photo-electric tube comprising an electron emitting surface, anelectrode parallel to said surordinary/A, etallic conduction.

`face and permeable to electrons, a second surface parallel to saidelectrode and electrically connected thereto, and means for deectingelectrons passing between said electrode and said second surface.

5. A photo-electric tube comprising an electron emitting surface and anelectrode permeable to electrons, a second surface electricallyconnected with said electrode, and means for deecting electrons -passingbetween said electrode and said second surface, said electrode andsecond surface being permeable to light.

6. A photo-electric tube comprising an anode screen, a light sensitivecathode positioned in front of said anode screen, and an auxiliaryelectrode positioned behind said screen, and having all except a minutearea shielded from said cath- 7. A photo-electric device comprising ananode screen, a light sensitive cathode positioned in front of saidscreen, means for forming a substantially equipotential space behindsaid screen, and an auxiliary electrode positioned to intercept aportion of a discharge entering said equipotential space from saidcathode.

8. A photo-electric apparatus comprising means for forming an opticalimage, means for forming a beam of electrons constituting an electricalimage corresponding to said optical image, an electrode for interceptinga portion of said beam, and means for varying the portion of the beamintercepted.

v 9. A photo-electric apparatus comprising means for forming an opticalimage, means for forming a beam of electrons constituting an electricalimage corresponding to said optical image, an electrode for interceptinga portion of said beam, and means for deecting the beam to vary theportion intercepted by said electrode.

10. Av photo-electric device comprising an anode screen, a lightsensitivel cathode positioned in front of said screen, means for forminga substantially equipotential space behind said screen, an auxiliaryelectrode positioned to intercept a portion of a discharge entering saidequipotential space from said cathode, and means for deecting thedischarge Within said space to vary the portion thereof intercepted bysaid electrode.

11. A photo-electric apparatus comprising a cathode capable of emittingelectrons in proportion to its illumination and positioned to permit theprojection of an optical image thereon, an electrode positioned toreceive the discharge from said cathode, and means for restricting thereceived discharge to that emitted from a minute area of said cathode.

12. Photo-electric apparatus comprising a light-sensitive cathodepositioned to permit the focusing of an optical image thereon, and aplurality of anode screens positioned in front of said cathode, wherebyelectrons from said cathode are accelerated to form an electrical imagecorresponding to said optical image.

13. Photo-electric apparatus comprising a light-sensitive cathodepositioned to permit the focusing of an optical image thereon, aplurality of anode screens positioned in front of said cathode, wherebyelectrons from said cathode are accelerated to form an electrical imagecorresponding to said optical image, and means for deecting saidelectrical image between the screens.

14. Photo-electric apparatus comprising a light-sensitive cathodepositioned to permit the focusing of an optical image thereon, aplurality of -anode screens positionedin front of said cathode, wherebyelectrons from said cathode are accelerated to form an electrical imagecorresponding to said optical image, and an auxiliary electrode havingall but a minute area shielded from said cathode and positioned tointercept an elementary area of said electrical image.

PHILO T. FARNSWORTH.

